I'm designing a product that will use the TPA3106D1 device, driving an 8 ohm speaker through a 24" cable.
I've created an intermediate prototype board for product developement, and on it, implemented the same circuit / layout / components as the TPA3106D1 EVM.
The challenge with my product design is that it requires the speaker lines (about 24" long) to share a cable and PCB connector with two Shure SM93 Microphones (Unbalanced, pre- pre-amp). I know that no audio engineer would recomend such a design, but in my case, I have no option and my job is to make it work as good as I can get it.
Being that the Amp's oscillator frequency is 400 Khz which is above the hearing range, This configuration actually works better than I had expected. However, when I power the amp, I can actually hear my mic preamps (located on the PCB a couple inches from the amp) react initially.. even though its not exactly audible. the compression kicks in a bit. When I listen closer, any time the amp's outputs are active (unmuted) the noise floor of my mic preamp output is higher. Its a "white noise". Putting a .001 capacitor from the signal lead of the mic to ground cuts that white noise down by about 70%. Any larger value capacitor there cuts it even further but starts to attenuate the high end response of the mic.
The only measurement equipement I have at my disposal is an Oscope. I can definately see the PWN outputs before the filter, and I can see a small amout (no longer square wave) remaining in the audio lines downstream of the filter. This is at 400Khz though. I cannot see whatever is creating audible white noise in my mic signal. I'm assuming it is some harmonic or something.
The TI amp dev board that I used as a reference seems to have two different output filters: an RC filter (which I calculated to have a cuttoff frequency of 4 Mhz not sure if my calculation is right), and the large LC filter.
Using that as a starting point, I'm looking for anything I could add or do to tame the output of my amp further. I'm not real strong when it comes to analog filters and RF.
Thanks